Stabilization of 3, 4-dihydro-1, 2-pyran-2-carboxaldehyde



Patented Jan. 9, 1951 STABILIZATION OF 3,4-DIHYDRO-L2- PYRAN 2 CARBOX'ALD EHYDE" Eugene E. Fountain and Walter H.. Sharp, Berke-- ley, Calif., assignorsto Shell Development Company, San Francisco, Calif., a. corporation of Delaware.

N Drawing. Application August" 9, 1949, Serial. No. 109,424

16. Claims. 1 This invention relates to a method of stabi1izing' 3,4"dihydro-l,2-pyrane2-carboxaldehyde to make possible its storage over prolonged periods of time, in the presence or absence of light; and to minimize the deleterious changes otherwise occurring therein under normal and usual conditions. of transportation and storage. The in.- vention relates particularly to the stabilization of 3,4-dihydro-1-,2pyran-2-carboxaldehyde to prevent spontaneous changes thereof. with time, particularly inthe nature of decreases withv time as during storage, of the carbonyl. content and the titre of the olefinic bond and to prevent or to minimize formation of Water-insoluble polymer. The invention. also relatesv to the stabilized compositions of matter resulting from the applica tion of themethod of. the invention.

The compound 3, l-dihydrolj-pyram2-carboxaldehyde, which can be produced by dimerization of acrolei-n and may be referred to as acrolein dimer, isa heterocyclic. unsaturated.

aldehyde having a structure. that may be represented by the formula When freshly prepared it is a clear water-white mobile liquid having av refractive index of about 1.463 to 1.466 and a boiling point'of about 88 C'.

under 100' millimeters mercury pressure and. about 146 C. under 760' millimeters mercury" autopolymeriaati'on of. 3-, i-dihydro-l.,2 -pyran-2- carboxaldehyde appears to proceed by at least. two separable types of change. The autopoly-- merization appears to be caused in part by the action of light and possibly by the action of peroxides, or both. Evidence for this conclusion is obtained from the fact.- that. the autopolymerization of. i'aedihydro-1,2-pyran-?r-carboxaldehydecan. be retarded either by adding thereto:

anefiectiveamcunt (generally from about 0.065%

ever,the 3,, l-dihydro-1,2-pyran-Z-carboxaldehydel still increase: in viscosity, albeit at a retarded ratel whether or not, the above precautions are. taken,,.

i until, after a number of months, it is completely.

and. irreversibly transformed. to an insoluble solid, The autopolymerizationof 3,4'-dihydro- -l,2-. pyran-2-carboxaldehyde which occurs in. the absence of actinic' light and is substantially un.-' aiiected by thepresenceof phenolic antioxidants,.' such as hydroquinone, appears to be used. upon. an obscure. mechanism. which, at. the present. time, is not Well understood. That. the mechae. nism does not involve the. intervention of, for. example, peroxides, appears. to be. convincingly shown by the ineifectiveness of hydroquinomd and related. antioxidants for preventing the auto-; polymerization. which. is caused by actinic light... It. appears. to. be quite unlikely that the non.- actirn'cally induced autopolymerization. involvesmerely the. formation. of. vinyl-type polymers or. of polymers formed via polymerization through the formyl group. It will be. noted that. the polymeric products of such autopol-ymerization. con-I tain. greatly reduced contents. of both the 016- finic bondand the-formyl group, indicating that both. of. these reactive groups. probably are in volved in. the polymerization reaction.

It. will be. evident that. such non-actinic autopolymerization. of 3,4.-dihydro-1,2-pyran 2rcar boxaldehyde highly disadvantageous because; it precludes economic storage of 3,4.-dihydro-1,2I-"f pyran-Z-carboxaldehyde for any prolongedv periods of time, even though. a phenolic antioxidant may have been added to reduce the. actinic: or. peroxideetype of polymerization. It j. thus hasbeen. necessary either to limit the time. of storage to an undesirably short time, with.- in. which excessive change does not occur, or to encounter; the risk of involving serious losses of product .-d'ue to irreversible conversion tothey viscous oreven solidmaterial formed under such. conditions. of: storage.

Itzis. a principal. object of the present: invention.- t P event or to substantially precludethenow.

actinic autopolymerization of 3,4-dihydro-1,2- pyran-Z-carboxaldehyde, which autopolymerization occurs in the presence or the absence of light and is incapable of being effectively prevented by the addition of phenolic antioxidants, such as hydroquinone and related mono-nuclear and di-nuclear phenols. It is a particular object of the invention to provide a stabilized composition of matter essentially comprising 3,4-dihydro-L2- pyran-Z-carboxaldehyde which can be stored alone or in admixture with other materials for long periods of time without the occurrence of undesired polymerization. A method for obtaining such novel compositions of matter is another object of the invention.

It has now been discovered in accordance with th present invention that the gradual change in or autopolymerization of 3,4-dihydro-1,2-pyran- 2-carboxaldehyde occuring over prolonged periods of time, particularly in the absence of light, can be prevented or effectively reduced by acidifying 3,4-dihydro-l,2-pyran-2-carboxaldehyde by addition thereto of a small amount of a low-molecular weight acidic substance, such as an acidreacting material or a material which liberates acid in situ. Very small amounts of such ac dic substances are effective. From as little as 0.0001% to as much as by weight of the 3,4-di-ydro- 1,2-pyran-2-carboxaldehyde can be employed, although even larger or smaller amounts may be used, if desired. In general, it is preferable to employ an amount of the acidic substance within the range of from about 0.001 to about 1% by weight of the 3,4-dihydro-1,2-pyran-2-carboxaldehyde. As an outcome of the invention, samples of the 3,4-dihydro-1,2-pyran-2-carboxaldehyde stabilized by the incorporation therewith of such traces of acidic substances can be stored in the absence of actinic light for periods of time up to many months without significant change, whereas otherwise identical samples containing no acid-reacting material and stored under the same conditions for an equal length of time undergo excessive and detrimental autopolymerization.

For the purposes of the present invention, an

acid-reacting material is defined as a material which has an acidic dissociation constant or contains at least one dissociable hydrogen atom having an acidic dissociation constant at least about '1 10- determined in an aqueous medium. Preferred acid-reacting substances are those that have at least one dissociable hydrogen atom or acidic group having an acid dissociation constant of at least about 1 10 It is desirable to employ as the acid-reacting substance or the material which reacts in situ to liberate acid, one which is devoid of reactive groups or atom which would combine with the 3,4-dihydro-L2-pyran-2- carboxaldehyde which is to be stabilized. In general, it is preferred to employ acid-reacting materials which are devoid of phenolic hydroxyl groups, that is of hydroxyl groups which are directly linked to an aryl group.

In accordance with the invention, various acidreacting, materials or materials which liberate acid in situ, may be employed to acidify 3,4-dihydro-l,2-pyran-2-carbox-aldehyde thereby stabilizing it against non-actinic autopolymerization. One group of acid-reacting substances comprises in particular the inorganic, preferably the strong mineral acids and most desirably the "strong non-volatile mineral acids. Illustrative of these are, for example, sulfuric acid, sulfurous acid, arsenic acid, phosphoric acid,the' hydrohalogen acids, including hydrochloric acid; hy-:

' of stability.

drobromic acid, and hydroiodic acid, tellurous acid, tungstic acid, and the like. In general it is preferred to employ a non-volatile strong mineral acid, such as phosphoric acid or sulfuric acid. Instead of a mineral acid there may be employed in accordance with the invention any of the various organic acids which have the characteristics hereinbefore and hereinafter described. Illustrative carboxylic acids which may be employed in accordance with the invention include, among others, the aliphatic, preferably saturated, monocarboxylic acids, such as formic acid, acetic acid, propionic acid, trichloroacetic acid, butyric acid, valeric acid, caproic acid, chloropropionic acid, bromobutyric acid, capryllic acid, capric acid, lactic acid, methoxyethcxyacetic acid, carbethoxypropionic acid, gamma-ethoxybutyric acid, epsilon-diethylaminovaleric acid, glycin hydrochloride, gamma-acetylbutyric acid, beta-butoxypropicnic acid, and the like, and homeless and analogs thereof. There also may be employed aromatic carboxylic acids such as benzoic acid, toluic acid, phthalic acid, isophthalic acid, trimesic acid, mellitic acid, nicotinic acid, acrydinic acid, phenylacetic acid, cinnamic acid and the like; as well as non-aromatic cyclic carboxylic acids such as cyclohexanecarboxylic acid, acid, cyclopentanecarboxylic arid, cyclohexanedicarboxylic acid-1,2, and heterocyclic acids, such as 2-pyrrolecarboxylic acid, quinaldic acid, antipyric acid, cinchoninic acid, furoic acid, dihydropyran-Z-carboxylic acid, and homologs and ana* according to the invention comprises the arc--- matic sulfonic acids, such a p-toluenesulfonic acid, benzenesulfonic acid, p-methoxybenzenesulfonic acid, beta-naphthalenesulfonic acid, 1,5- naphthalenedisulionic acid, 5-quinolinesulfonic acid, and the like. Sulfonic acids which contain a carboxyl group in addition to the sulfe group are exemplified by o-sulfobenzoic acid, l-sulfophenylacetic acid, 2-methyl-4-sulfobenzoic acid and 4-sulfo-2-chlorobenzoic acid.

While the invention includes generically the use of any carboxylic acid, which carboxylic acid may contain only one carboxyl group or may contain a plurality of carboxyl groups, it has been found that aliphatic saturated dicarboxylic acids are of particular eificacy for the obiects of the invention and that their use as acidic sub-' stances in accordance with the invention leads to stabilized 3,4-dihydro-1,2-pyran-2-carboxaldehyde compositions having an optimum degree Included in this particularly pre ferred group are acids of the general formula HOOC(CH2) a:-COOH wherein :c represents an integer, preferably one having a value of from 0 to 4, inclusive, and wherein one or more of the hydrogen atoms of the CH2 group or groups may be substituted by oxy or hydrocarbon groups, for example, alkyl. Included in this particularly preferred group of carboxylic acids are oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, tartaric. acid, pimelic acid, azelaic acid, brassylic acid; alpha. alpha'-dimethyladipic acid, alpha,beta-- ethoxysuccinic acid, malic acid, thiomalic aci d', beta-chloro-beta-methylmalic acid,.-;phenylsuc-,;

cyclohexaneacetic acid,"

assrawe:

contain at least one dissociables hydrogen: atom;-

or acid group having an acid dissociation con.- stant determined in aqueous: medium: of atv least txl'fland a molecular'weight not over about. 450', and. substances which generate. in situ". an acid-reacting material. containing: at least one dissociable hydrogen atom or acid. group having: an acid dissociation constant; determined; in

aqueous medium. of. at least. 1x10 and a molecular weight. not over about. 450; Itis. preferred; to employ acid-reacting materials: that; contain. a. minimum. of acid groups; generally not. overthree, per molecule, at: least: one: of said groups having an acid-dissociation constant of at least: 1- Particularly efficacious acid-reacting materials are those having-molecular-weights up to about 20.0.. Optimum results have been. obtained with. organic acids: which are devoid. oi phenolic: hydroxyl groups,. or hydroxyl groupsdirectly linked to an aryl group.

The acidifying or acid-reacting. substances may be added. to or mixed. with the. 3-.4-dihydro-l2- pyran-2-carboxaldehyde in' any convenient manner and at any suitable time prior tolorduring purification of. the aldehyde or prior to or;

during storage. It is particularly efficacious tomix the acid-reacting; material. in the desired amounts with the SA-dihydro-l.2-pyranF2-carboxadehyde shortly after: purification of the;

aldehyde since in this manner the product to hydro-1,2 pyran- 2-carboxaldehyde has a great,

afiinitv for water and the presence of even minor quantities of water tends to markedly accelerate the formation of polymers and other products For ordinary purposes the 3,4-dihydro-L2-pyran- Z-carboxaldehyde will be stored in the substantially pure state. Solutions of 3,4-dihydro-l,2--

pyran-Z-carboxaldehvde in suitable inert organic solve ts may be stabilired in accordance with the invention. The invention embraces the stabilization of compositions comprising or consisting of 3,4-dihydro-1 2:-pyran-2carboxaldehyde wherein it is desired to. retain the heterocyclic aldehyde in monomeric form free of polymers for prolonged periods of time. addition or incorporation of the acid-reacting substances, the SA-dihydro-1,2-pyran-2-carboxaldehyde may be retained or stored at any temperature consistent with the ordinary requirements of storage and transportation. The aldehyde preferab y is stored without'access thereto of actinic light, as in an. opaque container, a glass container that is. non-transmissive to= actinic light, or in a locality from which lightis excluded.

phenolic antioxidants or phenolic polymerizaticn inhibitors, such as" hydroquinone' and like mono-, diand trihydric. phenols. of the.'char-' acter referred to hereinbefora. may be incorpm Following the Independently of the practice of p the method of the present invention, suitable 6-. rated with the 3,i-dihydro--I.2'pyran-2l-c.arbox-- aldehyde to reduce orto prevent thepolymerization which is brought-about by exposure toactinic light, perhaps induced by peroxides; or peroxidesimulating compounds.

To illustrate the effectiveness of the method of the present invention and to present representative. examples: of stabilized novel: compounds provided thereby, the following examples are given.. It is to be understood that the examples are presented for the purpose of illustrating certain of the. specific embodiments of the principles of the invention and that they are not intended to be in the nature of limitations upon the invention as. it is more broadly defined in the. hereto: appended claims.

In the. following examples the experimentswere conduced by taking portions of freshly distilled samples of 3,4 dihydro-1,2-pyran-2-carboxaldehyde and mixing. therewith the desired amount of the acid-reacting substance to "be tested. Each sample then was stored at av constant temperature in stoppered glass bottles which were opaque to actinic light. In this manner possible interference resulting from actinic autopolymerizationv was avoided and the selective stabilizing action of. the materials incorporated with the 3,4-dihydro-1,2 pyran-2-carboxalclehyde; was demonstrated.

Example I To illustratethe autopolymerization of- 3,4-dihydro-1,2.-pyran-2-carboxaldehyde that occurs in the. absence of actinic light, a sample of 3,4

During 15 days. storage under. these conditions,

the refractive index of the unstabilized material increased by l09 10 during 27 days storage by 187 ,1-Q- and after 35 days storage the refractive index had increased by 231 10 At this time the carbonyl content of the sample was found to have decreased to 42% of the original carbonyl content. When stored at room temperature but under conditions othrwise the same, the refractive index of a further portion of the purified 3 ,4-dihydro-1,2-nyran-2-carboxaldehyde increased in 27 days by 235x 10- and the carbonyl content decreased to 30% of theory. 7

At the same time, there was mixed with a further portion of the freshly purified 3,4-dihydro-l,2-pyran-2-carboxaldehyde employed above, 0.02% by weight of oxalic acid and the stabilized sample was stored as above at 110 F. During 35 days storage under these conditions the refractive index increased only 30 1O units. At the endof this time the stabilized sample was found to have. a carbonyl content %of. theory,

compared to the original 97% of theory. After 124. days? storage the carbonyl content of the stabilized sample was 93% of theory and the sample was a fluidliquid, whereas an unst'abilized' sample stored under the same conditions: for thesame time was. convertedto a hard solid.v

Example If A. freshly purified sample. of 3,4l-dihydroeh2e pyranrz-carhoxaldehyde. having a, refractive in dex 01, of 1.4633 and a carbonyl value 98% of theory, was stabilized by the addition of 0.005% by weight of oxalic acid and the stabilized composition was stored in a closed brown bottle at 110 F. After 27 days storage it was found that the refractive index had increased only by 23 1O and that the carbonyl content was 96% of the original value. Under the same conditions, a further sample of the same freshly purified product which was stored without the addition of a stabilizer had a carbonyl content after 27 days storage only 43% of the initial value and its refractive index had increased by 150x10- units.

A still further portion of the same freshly purified material was stabilized by the addition of 0.02% by weight of oxalic acid and stored under the same conditions employed with the other two samples. After 27 days of storage, the refractive index of this sample increased by only 10X 10 units and the carbonyl content was 97% of theory compared to the initial 98% of the theory.

Example III To separate samples of freshly purified 3,4- dihydro-l,2-pyran-2-carboxaldehyde having a refractive index (n of 1.4633 and a carbonyl content 98% of theory there were added the indicated amounts of the materials listed in the tables below. The stabilized samples were stored at 110 F. in stoppered brown glass bottles, the refractive index of each sample being measured at the indicated times to determine the extent of change, if any, during the storage. At the conclusion of the experiments the carbonyl contents (as percent of theory) of the samples were analytically determined. The results shown in the table were obtained. In the tables, the columns headed nnxlo' show the increase in the refractive index at the indicated times and the columns headed :0 show the analytically determined carbonyl content of the sample at the indicated times, expressed asv percent of,

theory for pure 3,4-dihydrol,2-pyran -2-carboxaldehyde.

The claimed invention is:

1. As a new composition of matter, 3,4-dihydro-1,2-pyran-2-carboxaldehyde containing from about 0.001% to about 1% by weight of oxalic acid dissolved therein, said composition being stabilized against non-actinic autopolymerization.

2. As a new composition of matter, 3,4-dihydro-l,2-pyran-2-carboxaldehyde containing from about 0.001% to about 1% by weight of a saturated aliphatic dicarboxylic acid dissolved therein, said composition being stabilized against nonactinic autopolymerization.

3. As a new composition of matter, 3,4-dihydro-1,Z-pyran-Z-carboxaldehyde containing from about 0.001% to about 1% by weight of sulfanilic acid dissolved therein, said composition being stabilized against non-actinic autopolymeriza tion.

4. As a new composition of matter, 3,4-dihydro-1,2-pyran-2-carboxaldehyde containing from about 0.001% to about 1% by weight of an arcmatic sulfonic acid dissolved therein, said composition being stabilized against non-actinic autopolymerization.

5. As a new composition of matter, 3,4-dihydro-1,2-pyran-2-carboxaldehyde containing from about 0.001% to about 1% by weight of a lower saturated aliphatic monocarboxylic acid dissolved therein, said composition being stabilized against non-actinie polymerization.

6. As a new composition of matter, 3,4-dihydro-l,Z-pyran-Z-carboxaldehyde containing from about 0.001% to about 10% by weight of a carboxylic acid dissolved therein, said composition being stabilized against non-actinic autopolymerization.

7. As a new composition of matter, 3,4-dihydro-l,2-pyran-2-carboxaldehyde containing from about 0.0001% to about 10% by weight of a sulfonic acid dissolved therein, said composition being stabilized against non-actinic autopolymerization.

8. As a new composition of matter, 3,4-dihy- 5 dro-1,2-pyran-2-carboxaldehyde containing dissolved therein from about 0.0001% to about 10% Table I Time, in Days Amount, Polymerization Inhibitor g 7 27 35 Weight Aexim AnXl0 A'nX10 358 An 10 628 79 172 239 30 251 22 14 32 53 87 94 Trichioroacetic Acid O. 02 27 42 55 b6 78 82 Table II Time, inDays Amount, Polymerization Inhibitor g 15 35 123 Weight P C 11 P G t C er en er en Per ent AnXIO' 0:0 Aa 1o 0:0 Anxm 0:0

None 113 as 312 15 had solidified Phthalic Anhydride 0.02 10 97 29 93 125 64 Succinic Acid. 0.02 12 96 35 92 147 58 Tartaric Acid. O. 02 13 96 33 92 152 57 Suifaniiic Acid. 0. 02 i 4 98 I0 97 25 94 Anthranilic Acid 0. 10 77 Salicyclic Acid 0. 10 82 therewith from about 0.001% to by weight of an acid-reacting substance having an acidic dissociation constant of at least about 1x10 said composition being stabilized against non-actinic autopolymerization.

9. As a new composition of matter, 3,4-dihydro-1,2-pyran 2 carboxaldehyde having dissolved therein a minute amount of an acidic substance having an acidic dissociation constant of at least about 1 10 said composition being stabilized against non-actinic autopolymerization.

10. The method of inhibiting the non-actinic autopolymerization of 3,4-dihydro-L2-pyran-2- carboxaldehyde, which comprises incorporating about 1% by weight of oxalic acid.

11. The method of stabilizing 3,4-dihydro-L2- pyran 2 carboxaldehyde against non-actinic autopolymerization, which comprises incorporating therewith from about 0.0001% to about 10% by weight of a saturated aliphatic dicarboxylic acid.

12. The method of stabilizing 3,4-dihydro-L2- pyran-2-carboxaldehyde, which comprises adding thereto from about 0.001% to about 10% by weight of a lower saturated aliphatic monocarboxylic acid.

13. The method of inhibiting the autopolymerization of 3,4-dihydro-1,2-pyran-2-carbox- 7 subsequent storage inaccessible to actinic light.

aldehyde, which comprises incorporating there- 30 with from about 0.001% to about 1% b weight of sulfanilic acid.

14. The method of stabilizing 3,4-dihydro-L2- 16. The method of storing 3,4-dihydro-L2- pyran-Z-carboxaldehyde for relatively, prolonged periods of time under conditions normally conducive to autopolymerization of 3,4-dihydro-1,2- pyran-Z-carboxaldehyde, which comprises incorporating with said 3,4-dihydro-1,2-pyran-2- carboxaldehyde a minute amount of an acidifying substance having a dissociation constant of at least 1 10-".

EUGENE B. FOUNTAIN.

WALTER H. SHARP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,479,283 Whetstone Aug. 16, 1949 2,480,990 Whetstone Sept. 6, 1949 

8. AS A NEW COMPOSITION OF MATTER, 3,4-DIHYDRO-1,2-PYRAN-2-CARBOXALDEHYDE CONTAINING DISSOLVED THEREIN FROM ABOUT 0.0001% TO ABOUT 10% BY WEIGHT OF AN ACID-REACTING SUBSTANCE HAVING AN ACIDIC DISSOCIATION CONSTANT OF AT LEAST ABOUT 1X10-7, SAID COMPOSITION BEING STABILIZED AGAINST NON-ACTINIC AUTOPOLYMERIZATION. 